1. Field of the Invention
The present invention relates to a method and system for mobile station location estimation in which base stations that are in line of sight of the mobile station and base stations that are not in the line of sight of the base station can be determined. Errors in base station signals generated from determined non-line of sight base stations are reduced for providing improved mobile station location estimation.
2. Description of the Related Art
Mobile location estimation determines a geographical estimate of the location of a mobile station. Mobile location estimation is useful in management of fleets of mobile stations, location dependent information services, location dependent billing services and Emergency 911 location of a mobile station. Enhanced 911 is designed to automatically forward the number of a caller to a public safety answering point (PSAP). In implementing enhanced 911 in a wireless network, wireless service providers provide two dimensional location of the vehicle to the public safety answering point (PSAP). The Federal Communications Commission (FCC) has regulated by the year 2001 that wireless service providers have the capability of locating callers in two dimensions within 125 meters 67% of the time.
One conventional method for locating a mobile station in two dimensions would use the measurement of the line of sight distance between the mobile station and at least three base stations. U.S. Pat. No. 5,365,516 describes a method for determining the location of a transponder unit in which a radio signal is sent by the mobile station. The arrival time of the radio signal is measured at each of three base stations. Each distance measurement between the mobile station and one of the base stations can be used to generate a circle which is centered at the measuring base station. The circle has a radius which is equal to the distance between the mobile station and the base station. Accordingly, three circles are generated, one for each of the base stations. In the absence of any measurement error of the distance between the base stations and the mobile station, the intersection of the three circles unambiguously determines the location of the mobile station. This method has the drawback that the distance measurements can be corrupted by noise resulting in errors in determining the location of the mobile station.
A conventional solution for providing more accurate position estimates is to reduce the error due to noise with a least squares analysis. Accordingly, the least squares analysis provides a more accurate position estimate. This solution has the limitation of not accounting for the possibility of a lack of a direct path between the base station and the mobile station. For example, in an urban environment, a building or buildings may be in the path between the mobile station and the base station. A propagating signal between the mobile station and the base station can be reflected and defracted by the object in the path of the mobile station to the base station resulting in the signal traveling excess path lengths. The excess path lengths can be on the order of a hundred meters.
The lack of direct path between the mobile station and the base station can be defined as a non-line of sight (NLOS). The importance of detecting and reducing the NLOS measurements between a mobile station and a base station is recognized in M.I. Silventoinen, et al., "Mobile Station Locating in GSM" IEEE Wireless Communication System Symposium, Long Island N.Y., November 1995 and J. L. Caffrey et al., "Radio Location in Urban CDMA Microcells", Proceedings of the Personal, Indoor and Mobile Radio Environment, 1995.
U.S. Pat. No. 5,365,516 ('516 patent) describes an embodiment of a transreceiver locating system operating in an environment susceptible to multipath interference. The system includes a transponder which is operable within a prescribed coverage area to transmit a burst of data symbols in a coded carrier pulse. Each base station includes a receiver for detecting and responding to the data symbol at a given time, interrupting the data symbol and rejecting echoes resulting from multipath interference. A comparison circuit responds to the receiver for comparing respectively identified given times and decorrelating the time difference to improve data quality. Although the '516 patent addresses multipath interference, it does not attempt to detect base stations for reducing multipath NLOS with mobile stations.
It is desirable to provide a method and system for providing improved mobile location estimation which is robust to NLOS error.